Sinkhole risk in Grafton County, NH: Mostly deep or small

Mostly deep or small

Some soluble rock, mostly deeply buried or in a small share of the county.

On soluble rock: 2%
At or near the surface: 0%
Buried under cover: 2%

What this means before you buy

Sinkholes are uncommon here; watch for new depressions after heavy rain or leaking pipes.

That works out to roughly 29 of Grafton County's 1,709 square miles on soluble rock. With about 93,045 residents (54 per square mile), much of the land is open, and the mapped rock may sit far from where people actually live. The figures come from 606 map points about 1.7 miles apart, so a pocket smaller than that can be missed or overstated.

Worth reading for this county: how these percentages are measured, and the warning signs that apply anywhere.

Counties around Grafton County

The nearest county rated sinkhole country is Union County, PA, about 335 miles southwest. 1 of the 6 closest counties land in the same band; the ground changes quickly around here.

Orange County, VT 27 mi west · 0% at surface
Carroll County, NH 32 mi east · 0% at surface
Belknap County, NH 35 mi southeast · 0% at surface
Caledonia County, VT 40 mi north · 0% at surface
Sullivan County, NH 43 mi southwest · 0% at surface
Merrimack County, NH 44 mi south · 0% at surface

The rock underneath

The largest mapped soluble unit here is the Fitch and Clough Formations, undivided (quartzite), Silurian in age, covering about under 1% of the county. Next is the Fitch Formation (Upper Silurian; Pridolian and Ludlovian) at under 1%.

Against the rest of New Hampshire

Grafton County ranks 5 of 10 in New Hampshire for soluble rock at the surface, and sits above 0% of US counties on that measure.